The invention relates generally to improving the performance and reducing the heavy metal content of PVC compounds by the partial or total substitution of conventional mixed metal stabilizers with phosphite esters, or blends thereof, with an effective amount of added zinc.
The PVC industry began with the invention of plasticized polyvinyl chloride (xe2x80x9cPVCxe2x80x9d) by Waldo Semon of the B. F. Goodrich Company in 1933 as an alternative to natural rubber where its non-flammability made it ideal for wire insulation, particularly on naval ships. However, unlike rubber, PVC has a tendency to discolor and is not easy to process well. Stabilization is required to perform two basic functions: (1) prevent discoloration; and (2) absorb hydrogen chloride (HCl) which evolves during process. It is believed that billions of pounds of flexible PVC are employed throughout the world in a wide variety of commercial applications. These include vinyl flooring, wall covering, roofing, pond and pool liners, film, upholstery, apparel, hose, tubing and wire insulation.
In order to successfully process vinyl compounds into finished vinyl articles by extrusion, calendering or molding, it is necessary to incorporate between one and five percent of a heat stabilizer to prevent dehydrohalogenation and discoloration of the polymer during thermal processing. The preferred vinyl heat stabilizers for most flexible PVC applications in the United States are referred to as xe2x80x9cMixed Metalxe2x80x9d heat stabilizers. They are complex multi-component chemical admixtures based upon combinations of alkaline earth and heavy metal salts with a variety of antioxidants, HCl absorbers and chelating agents. The most widely used mixed metals are based upon and referred to as Barium-Cadmium, Barium-Cadmium-Zinc, Barium-Zinc and Calcium-Zinc stabilizers. However, mixed metal heat stabilizers suffer from several drawbacks. If the level of zinc is too high, the polymer will char very rapidly. Additionally, barium and cadmium are toxic heavy metals which while they do provide heat stability, their presence adversely affects clarity, plate out and stain. In order to counteract these negative effects, further additional components were blended into the formulations, making PVC additive formulation and processing a highly unique and specialized art. Clearly, what was needed was an approach which used higher performance phosphites and added back only what was needed.
Phosphites can stabilize the PVC in a variety of mechanisms. They have been shown to scavenge HCl and replace labile chlorine (via the Arbuzov rearrangement), add to double bonds, decompose peroxides and complex Lewis acids. The need for heavy metals such as cadmium and barium can be eliminated by choosing the correct phosphite and optimizing the level of zinc in the PVC formulation. The phosphites were chemically designed to build mechanism into the molecule itself to perform the various functions required to stabilize flexible PVC. The resulting complex phosphite esters are liquid stabilizers, designed to be used either as partial or full replacements of mixed metal stabilizers in conjunction with small, but effective amounts of zinc.
This invention illustrates the manner in which various phosphite esters and phosphite ester blends can be used to augment, and replace, partially or completely, heavy metal components of mixed metal vinyl heat stabilizers, especially for both flexible and rigid PVC.
The invention discloses specific phosphite esters which when used in conjunction with small carefully controlled quantities of active zinc compounds, can replace partially or completely conventional high performance state of the art commercial mixed metal heat stabilizers on a part for part basis, while not only retaining, but actually improving the high heat stability and performance of the barium cadmium zinc or barium zinc products. Concurrently, and also surprisingly, the direct replacement of the barium cadmium zinc and barium zinc based stabilizers with the phosphite based compositions of the invention improve a multitude of ancillary and final properties of the PVC compounds.
The invention is directed to an additive composition which substantially reduces the content of mixed metal stabilizers based upon the toxic metals of barium and cadmium, by replacing those stabilizers with essentially toxic metal free phosphite ester compositions disclosed herein with small (catalytic, in the ppm range) amounts of zinc.
It is another object of this invention to improve certain ancillary properties of all mixed metal stabilizers, including calcium zinc types, with regard to their ability to improve initial color retention, clarity, plate out resistance, volatility, printability and compatibility of flexible PVC compounds by methods disclosed herein.
It is yet another object of this invention to disclose a method of totally replacing mixed metal stabilizers with essentially toxic metal free phosphite ester compositions disclosed herein with small (catalytic) amounts of zinc.
These and other objects of the present invention will become more readily apparent from a reading of the following detailed description taken in conjunction with the accompanying drawings wherein like reference numerals indicate the parts and appended claims.